| Current differential protection is widely used in the line protection and bus-bar protection for its simple principle, high reliability and high speed. And now it is one of the most important protective relaying to ensuring the safety and stabilization of power system. But it has two problems in applying: the first is that the electromagnetism TA saturation may easily result in its mal-operation; and the other is that the transmission line's distributed capacitive currents to ground make the currents at the two sides not satisfy the Kirchhoff's current law (KCL), which may also result in its mal-operation, especially in EHV long lines. And these will badly threaten the safety and stabilization of the power system.Aiming at the first problem, microprocessor-based protective relaying adopts lots of methods to distinguish TA saturation moment and eliminate TA saturation'effect, which make its arithmetic and criterion more complex and lower its rapidity, sensitivity and reliability. The OCT has no saturation and has high ability of being free of the electromagnetism interfering, which could solve this problem radically. Aiming at the other problem, though adopting kinds of compensating methods, it could not solve radically the problem of the traditional line fundamental current differential protection's performance degradation due to the line distributed capacitive current to ground. Thus studying the new principle or the new way of realizing current differential protection is the leading way.Using Laplace transform calculates the basic non-periodic currents at two ends of HV long transmission line under distributed parameter model. Analyzing and studying indicate that basic non-periodic currents at two ends are affected by the fault occurring moment in both initial magnitude and direction, and behave as fault currents in line internal fault whereas traversing currents in line external fault, which could distinguish line external fault from internal fault via calculating and comparing. Based on the OCT, which could measure the non-periodic current accurately, for the first time a novel line percentage restraint differential protection and a novel line directional differential protection based on the non-periodic current are proposed with their criterions, and an compare is make in their performances. When being used in line protection, the non-periodic current differential protection has a distinct advantage that it is not affected by the line distributed capacitor to ground, which could solve the puzzle of thetraditional line fundamental current differential protection's performance degradation due to the line distributed capacitive current to ground. Analyzing and simulation indicate that this novel protective relaying is correct and reliable in different working conditions of power system.The non-periodic current is related with the fault's occurring moment and is zero in some particular fault, whereas the travelling wave is the maximum. Thus for the first time an integration of the line non-periodic differential protection and the line travelling wave differential protection is proposed, which could solve the problem of low sensitivity in particular fault for both. If they are integrated with the traditional line fundamental current differential protection as an intelligent protection, performances of the line protection will be better.OCT has also the characteristic of digital output and network transmission, thus it could be connected directly with the microprocessor based protection and controlling device, which could realize sharing the data for all the station. All these will deeply affect the revolution of station automation system and will accelerate the development of digital substation. This paper make a pilot study of substation automation system based on OCT, and study the transformer overload cutoff system in detail which has multi-step and multi-order strategy, which has been widely used in the substation.
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